It is not unusual for optical systems (such as fiber lasers, fiber optic communications, and medical, industrial and remote sensing applications which utilize light delivery means) to handle high optical power signals. For example, it is common for such systems to utilize optical signals in a single fiber or waveguide having optical signal power of up to several Watts. These fibers and waveguides support a large variety of operating modes, for example, single-mode, multi-mode and polarization maintaining mode.
These optical systems suffer from noise which is commonly caused by optical reflections resulting from discontinuities. As an example, the end of an optical fiber may cause an optical signal to reflect and travel back towards the source. This reflected signal may interfere with the source operation. Additionally, the generated noise may limit the ability of system components to detect transmitted signals. It is known in the art to control reflection through the use of optical terminators which possess low reflection characteristics. These optical terminators typically utilize thin film coatings, optical absorbing polymers and adhesives, and optical black coatings. However, when the specific intensity or power per unit area of the terminated optical signal is relatively high, these prior art terminators are exposed to light fluxes beyond their damage thresholds and eventually fail. A need accordingly exists for an optical terminator that is capable of terminating or dumping optical signals having high intensities or powers. There would be an advantage if such an optical terminator could be placed at the end of the fiber line or integrated within an optical device.
It is further recognized that many optical systems are being designed to carry optical signals over a relatively broad spectral range. Unfortunately, many prior art optical terminators, in addition to being limited in terms of power, are further restricted to providing effective termination to a limited range of wavelengths. There is accordingly a need for an optical terminator that is capable of terminating a wide range of wavelengths (for example, from the visible at about 400 nm to the infared at about 2000 nm).
It is further recognized that transmitted optical signals can utilize any one of a number of selected transverse modes. The variety of mode choices include, for example, single-mode of various numerical apertures, multi-modes of various numerical apertures and polarization maintaining waveguides of various geometries. It would be an advantage if the optical terminator were configured to be capable of terminating a wide range of numerical apertures and mode structures.
There exist a number of known ways for realizing an optical terminator.
In accordance with one method, one or more highly absorbing materials are attached to the end of a fiber or a waveguide thus creating a spot of high temperature at the point of optical signal impingement. This solution puts the maximal limit of operation at the damage threshold of the chosen absorbing material. The damage threshold is limited since heat transfer times are slow, and in many cases not sufficiently fast to cool down the hot spot before damage, like melting or phase change, occurs to the absorbing media.
In another method, a higher power optical terminator is realized by using long volume absorbers in the core (for example, core absorbing fibers ATN-FB by CorActive Inc. Quebec, Canada). This solution is relatively expensive, and undesirably leaves the heat in the core volume which is generally small compared to the clad and cover cap volume. Additionally, these fibers usually can perform an absorbing function within a limited wavelength region.
Yet another method performs the absorbing function in a tight clad coated with polymer. Here, the core is non-absorbing core, with absorption occurring in a tight clad coated with polymer solutions. Undesirably, the core can nonetheless become heated by heat conduction from the absorbing tight clad, thus limiting the useful power range of the terminator to low powers. This solution requires a very precise matching of the core indexes (so as to prevent back reflection at the interface). Matching in this manner is a difficult task at a wide range of temperatures of operation since different materials used possess different dn/dT values (index change with temperature).
In summary, optical terminators which are better at handling higher power signals without damage, are capable of handling wider spectral ranges and further support many mode structures simultaneously, are needed. The present invention addresses these and other needs in the art.